The frequency spectrum of an external system used in the simulation of electromagnetic transients shows many peaks due to resonance effects of the leading transmission lines. Therefore, the two-level equivalent we developed contains simplified lines for the leading part of the system and lumped elements represented by rational functions as a correction for the rest of the external system. This leads to a computationally more efficient model, over a wide frequency range, than the existing

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... ents with only lumped parameter components. The paper describes how the parameters of the new equivalent are optimized for most accurate fitting of the input admittance while assuring its positive realness and accuracy at dc and power frequency. Generators in the external system are represented in the equivalent as Norton current sources at the connecting terminals. The equivalents can be single or multiport. Index Terms-External equivalents, multiport system, Norton equivalent, positive realness, quadratic programming, rational approximation, sequential quadratic programming.